drivers/net: avoid some skb->ip_summed initializations

fresh skbs have ip_summed set to CHECKSUM_NONE (0)

We can avoid setting again skb->ip_summed to CHECKSUM_NONE in drivers.

Introduce skb_checksum_none_assert() helper so that we keep this
assertion documented in driver sources.

Change most occurrences of :

skb->ip_summed = CHECKSUM_NONE;

by :

skb_checksum_none_assert(skb);

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/acenic.c b/drivers/net/acenic.c
index b9a59160..41d9911 100644
--- a/drivers/net/acenic.c
+++ b/drivers/net/acenic.c
@@ -2033,7 +2033,7 @@
 			skb->csum = htons(csum);
 			skb->ip_summed = CHECKSUM_COMPLETE;
 		} else {
-			skb->ip_summed = CHECKSUM_NONE;
+			skb_checksum_none_assert(skb);
 		}
 
 		/* send it up */